The present invention is directed to bicycles and, more particularly, to a shift control apparatus that prevents undesirable chain angles.
Some bicycles are equipped with shift control devices that automatically control the operation of front and rear transmissions (e.g., front and rear derailleurs that respectively cooperate with a plurality of front and rear sprockets) depending on bicycle velocity. An example of such a system is disclosed in JP 8-501,742. In that system, the bicycle chain may be engaged automatically with any combination of front and rear sprockets depending upon the velocity of the bicycle. The use of all sprocket combinations better accommodates more sensitive changes in riding conditions than is possible by operating the rear transmission alone and also avoids unnecessary shifting.
In the case of external transmissions that employ front and rear derailleurs and a corresponding plurality of sprockets for each derailleur, the front sprocket having the greatest number of teeth typically is the laterally outermost sprocket (along the crankshaft axis), with the number of teeth on the other sprockets progressively declining as one moves axially inward. On the other hand, the rear sprocket having the greatest number of teeth typically is the laterally innermost sprocket (along the hub axis), with the number of teeth on the other sprockets progressively declining as one moves axially outward. As a result, a chain engaging the laterally outermost front and rear sprockets provides the highest gear ratio, and a chain engaging the laterally innermost front and rear sprockets provides the lowest gear ratio.
Since conventional automatic bicycle transmissions use all sprocket combinations to accommodate many riding conditions and to avoid unnecessary shifting, the chain sometimes will engage the front and rear sprockets having the greatest number of teeth as well as the front and rear sprockets having the fewest number of teeth. The same is also true with manual shifting. These front/rear sprocket combinations represent combinations in which the chain engages an axially outermost sprocket and an axially innermost sprocket, and each causes a substantial longitudinal inclination of the chain with large angles of approach and departure relative to the sprockets. Such configurations result in poor transmission efficiency from the front sprocket to the chain and from the chain to the rear sprocket. Substantial noise and wear on the chain and sprockets also result, especially if the chain is not properly lubricated.